1. Field of the Invention
The invention relates to the field of controlling peak levels of frequency modulated (FM) signals.
2. Prior Art
Audio signals are frequently clipped at predetermined thresholds particularly in the broadcast field to prevent over-modulation of the transmitter, which is ordinarily defined by government regulations. In many instances, the xe2x80x9cclippingsxe2x80x9d are used in subsequent processing. The use of xe2x80x9cclippingsxe2x80x9d for signal processing is described in U.S. Pat. No. 5,737,434 entitled xe2x80x9cMulti-Band Audio Compressor with Look-Ahead Clipperxe2x80x9d and U.S. patent application Ser. No. 09/216,345, filed Dec. 18, 1998, entitled xe2x80x9cOversampled Differential Clipper,xe2x80x9d assigned to the Assignee of the present invention.
When this technology is applied to frequency modulated (FM) composite signal, unexpected difficulties arise in trying to create an FM composite clipper that is bandlimited to 53 kHz and that protects the 19 kHz pilot tone. Filtering the clipped signal causes overshoots greater than 10% modulation and also unduly compromises the stereo separation.
As will the be seen, the present application provides a new, non-linear approach to controlling the composite waveform that greatly ameliorates prior art disadvantages and produces a waveform that overshoots, in worse case, approximately 1.5% and is tightly bandlimited to 53 kHz.
A method for controlling the peak levels of an FM composite signal is described. First, the FM signal is clipped to obtain an error signal. This signal comprises the xe2x80x9cclippingsxe2x80x9d resulting from the clipping operation. Next, the peaks of the error signal are determined. Then, a half-cosine wave is fitted to each of the peaks. The resultant half-cosine-interpolated signal is filtered to provide a filtered error signal. This error signal is subtracted from the FM composite signal. In one embodiment, the filter includes a notch at approximately 19 kHz. In another embodiment, the FM composite signal is sampled asynchronously with respect to the 38 kHz FM signal and the peaks are found by interpolating between samples and then the 38 kHz half-cosine is fitted to each of the interpolated peaks.